The present invention pertains to telephones, facsimile machines, computer modems, automatic alarm dialers, and similar equipment, and specifically to detection apparatus in such equipment for ascertaining whether a telephone line is in On-Hook or Off-Hook condition.
A residential or business user might connect more than one device to one common telephone line. When one of the devices has an auto-dialing function, such as in a facsimile machine or a modem, problems will arise if the telephone line is in use (Off-Hook) when the automatic device attempts to dial a number. The automatic device thus should be able to detect whether the telephone is Off-Hook or On-Hook before dialing is attempted. Many telephone companies will not allow the use of automatic dialing equipment on their lines unless they contain line state detectors that block dialing attempts when the line is Off-Hook.
A telephone line in On-Hook condition has no current flow, and the line voltage is high, typically 50 V DC. A telephone line in Off-Hook condition sees a current consumption of 10-50 mA DC, and the line voltage is low, about 1-10 V DC. The line state can thus be detected either by sensing the current in the line, or by sensing the line voltage.
Devices other than ordinary telephones must have a line interface between the telephone line and the data circuits in the device. Such a line interface must provide a proper impedance match between the telephone line and the data circuits in the device, and it must also separate the data circuits galvanically from the telephone line. The galvanic separation is achieved by the use of transformers, relays or opto-couplers, or by a combination of such components.
Most of the presently used devices for detecting the state of a telephone line rely on sensing the current in the telephone line, usually by connecting a relay directly in series with the telephone wires. The relay coil must have low resistance, typically max 20 ohm, in order not to cause too much voltage drop in the line, and the relay must close at a current of 10 mA DC or less in the coil. The relay coil must also be shunted by a large bipolar capacitor, typically about 50 uF, to avoid attenuation of high frequency signals. Some telephone companies specify that the relay must have two coils, each with a shunt capacitor, in order to maintain symmetry on the telephone line. This known line state detector works well, but the components are expensive and bulky.
The current in the telephone line could also be sensed by means of an opto-coupler instead of a relay, but this introduces a constant voltage threshold of about 1 V in the telephone line, which is not acceptable to many telephone companies. Therefore, this method is rarely used.
A line state detector could also be based on sensing of line voltage instead of line current. Such a voltage sensing line state detector is described in U.S. Pat. No. 4,647,723. This device needs an amplifier in the voltage sensing circuit, because most telephone companies do not allow more than a few microampere drain on the telephone line in On-Hook condition, and such a low current can not directly drive an output device providing galvanic separation. The amplifier needs power, which must be galvanically separated from all circuits outside the voltage sensing circuit, so a battery is included as power source. The known voltage sensing line state detector is complicated and expensive, and the battery is a bulky and inconvenient component.